Traditional 1.047 .mu.m or 1.064 .mu.m laser wavelengths have been employed for more than 20 years to explosively remove laser severable circuit links to disconnect, for example, a defective memory cell and substitute a replacement redundant cell in a memory device such as a DRAM, an SRAM, or an embedded memory. Similar techniques are also used to sever links to program a logic product, gate arrays, or ASICs. FIG. 1A shows a conventional infrared (IR) pulsed laser beam 12 of spot size diameter 14 impinging a link structure 16 composed of a polysilicon or metal link 18 positioned above a silicon substrate 20 and between component layers of a passivation layer stack including an overlying passivation layer 21 and an underlying passivation layer 22. Silicon substrate 20 absorbs a relatively small proportional quantity of IR radiation, and conventional passivation layers 21 and 22 such as silicon dioxide or silicon nitride are relatively transparent to IR radiation.
To avoid damage to the substrate 20 while maintaining sufficient energy to process a metal link 18, Sun et al. in U.S. Pat. No. 5,265,114 proposed using longer laser wavelengths, such as 1.3 .mu.m, for processing links 18 on silicon wafers. At the 1.3 .mu.m laser wavelength, the absorption contrast between the link material and silicon substrate 20 is much larger than that at the traditional 1 .mu.m laser wavelengths. The much wider laser processing window and better processing quality afforded by this technique has been used in the industry for about three years with great success.
The IR laser wavelengths have, however, disadvantages: the coupling efficiency of an IR laser beam 12 into a highly electrically conductive metallic link 18 is relatively poor; and the practical achievable spot size 14 of IR laser beam 12 for link severing is relatively large and limits the critical dimensions of link width 24, link length 26 between contact pads 28, and link pitch 30. The IR laser link processing relies on heating, melting in link 18, and creating a mechanical stress build-up to explosively open overlying passivation layer 21. The thermal-stress explosion behavior is somewhat dependent on the width of link 18. As the link width becomes narrower than about 1 .mu.m, the explosion pattern of passivation layers 21 becomes irregular and results in an inconsistent link processing quality that is unacceptable.
The practical achievable lower laser spot size limit, accounting for selection of optical elements and their clearance from substrate 20, for a link-severing laser beam 12 can be conveniently approximated as twice the wavelength (2.lambda.). Thus, for 1.32 .mu.m, 1.06 .mu.m, and 1.04 .mu.m laser wavelengths, the practical spot size limits for link removal are diameters of roughly 2.6 .mu.m, 2.1 .mu.m, and 2.0 .mu.m, respectively. Since the lower limit of usable link pitch 30 is a function of laser beam spot size 14 and the alignment accuracy of laser beam 12 with the target location of link 18, the lower spot size limit directly affects the density of circuit integration.
The smallest focused material-removing laser spot size 14 currently used in the industry for repairing 64 megabit DRAMs is about 2 .mu.m in diameter. A spot size 14 of 2.1 .mu.m is expected to be useful through 256 megabit and some 1 gigabit DRAM designs. FIG. 2 is a graph of spot size versus year, demonstrating industry demands for smaller spot sizes as the link pitch 30 and link width 24 decrease. The graph is based on a simple formula for approximating spot size demands: spot size diameter=2(minimum link pitch)-2(system alignment accuracy)-(link width). (These parameters are shown in FIG. 1B.) The graph assumes 0.5 .mu.m accuracy through the year 1997, 0.35 .mu.m accuracy through the year 1999, and 0.25 .mu.m accuracy thereafter. Accordingly, industry experts predict that spot sizes under 2 .mu.m will soon be desirable for processing links 18. These spot sizes are not, however, practically achievable with conventional IR link-blowing laser wavelengths.
Shorter visible wavelengths, such as 0.532 .mu.m, would permit a reduction in the laser beam spot size. However, at these wavelengths the silicon substrates 20 are strongly absorbing and the laser link-severing process would damage part of the substrate 20. Substrate damage is unacceptable for the sake of ensuring reliability of the processed device.
What is needed, therefore, is a processing method and apparatus for severing electrically conductive links fabricated on a semiconductor wafer with selected laser wavelengths that reduce the practical beam spot size to considerably less than 2 .mu.m but do not damage the semiconductor wafer substrate while severing the links.